Why is the label black and silver on Riverside LP

Macallan,
The differant coloured labels were simply chosen during that time period.
I believe blue on white labels were the early start up colours.
By the mid 1950s a logo was introduced on the label of recording tape and reels centred by a microphone.

Black labels with the tape and microphone were stereo release,late 1950s.

From there to 1964 their label colours changed again.

The art work on the labels was changed over time.This fortunately allows us to determine the pressing's vintage by using release year/label art comparison.Original 1954-56 up to a number in the 240s are on The White label with Blue ink.From 56-65 the Original Mono issues are on a Blue label with Silver ink.The Logo featuring the 2 reels split by a microphone is above label name.From 1958-65 Original Stereo issues are on the Black Label with identical artwork.In 1965-6 the art changed to a blue label/silver print no logo.The name Riverside is in a thin typeface.All issues to this point must say "Bill Grauer Productions" at bottom of Label.From 1966-7 the Teal/Turquoise label with "Orpheum Productions" at bottom is largely re-ish.1968-71 gives us The Brown and Black with White lettering.Riverside inactive from 72-80 revived by Fantasy in 80s with all that dough they made off that Fogerty Boy.The new (re-ish) label is White with Gray artwork.Has the Reels and Mic but in a reverse configuration.The Catalog Prefix is either OJC (original Jazz Classic) OBC (Original Blues Classic) and Fantasy Records blurb is down where "Bill G. Prods." used to be.The point in all of this of interest to the collector is..Say you have a copy of ""Monks Music with 'Trane and The Hawk.The catalog # is 12-242 if it is original it's on the White with Blue ink "Bil G Prod." on bottom.The same cat # but on the Blue with Silver (reels & Mic plus Bill G Prod.) would be an early Mono re-ish (But yet still so lovely).If that same record were on a Black label with silver the cat # would be 9-1102 and it would be an early Stereo re-ish (and yes lovely as well).